✨ Add the SEO surface and refresh the landing page

Guides, directory collections, per-audience use-case pages, llms.txt and a pages sitemap, with breadcrumbs, FAQ schema and shared CTA partials. Landing page: the directory count is read from the database everywhere instead of a hardcoded 46 (new directory_count helper, %{directory_count} placeholders in use-case copy), the hero URL field gets a visible icon and a gradient focus ring, and the resources section is redesigned as a content section with a heading, blurbs and arrowed links instead of footer-style link columns.